Yield enhancement to increase crop production is one of the essential strategies to meet the demand for food by our continually growing populations. Abiotic stress limits crop production significantly by, e.g., reducing overall crop yield or decreasing crop quality. Abiotic stresses are environmental factors such as drought, salt, extreme temperatures and high winds. While many agricultural practices are designed to optimize crop growth by reducing or avoiding such abiotic stresses, improvements are still needed.
Soybean, one of the most important cash crops, is classified as a moderately NaCl-tolerant plant (Maas & Hoffman, 1977). Understanding the NaCl tolerance mechanism in this crop plant may ultimately help improve its yield on saline lands, and the NaCl tolerance conferring genes in soybean may also be applicable to crops that are more sensitive to NaCl (e.g., carrot, orange and rice). Salinity (accumulation of NaCl) is a severe abiotic environmental stress affecting plant growth. The adverse effects are multifaceted and include ionic stress, physiological drought, specific ion toxicity, and salt-induced oxidative stress. NaCl stress and dehydration stress were long thought to be closely associated since accumulation of salt (mainly NaCl) presents in external environment will lead to a drop of external water potential, which then become lower than that of the water potential in the plant cells. As a result, the net water flow will be from interior of the plant cell to its exterior. This leads to osmotic desiccation (Ashraf, 1994), also known as physiological drought in plants (Jain, et al., 1997).
RD22, first discovered in Arabidopsis, is induced by salt stress, but not by cold or heat stress (Yamaguchi-Shinozaki and Shinozaki, 1993). It was found that the plant hormone abscisic acid (ABA) is crucial for the induction of RD22 under dehydration and salt stress, although RD22 lacks the ABA-responsive element (ABRE) (Xiong, et al., 2001). It appears that dehydration triggers the production of ABA, in turns induces expression of various genes, cis- and trans-acting factors. Based on the current model for RD22 induction proposed by Abe and associates (1997), under osmotic stresses (including salt and dehydration stress), ABA is synthesized and triggers the production of RD22BP1 and ATMYB2 proteins, which act as transcriptional activators to bind to the MYC and MYB sites locating on a 67 bp fragment of the RD22 promoter, and hence activates the expression of RD22. However, while the regulation of gene expression of RD22 related to dehydration and salt stress were well studied, its precise role in plant adaptation to salt and dehydration stress remains unknown.
In sum, while gene expression has been correlated to abiotic stresses, a demonstrated ability of such genes to confer tolerance or resistance to such stressors is still lacking. Identification of genes having the ability to significantly alter tolerance to abiotic stress would represent a significant advance for genetic engineering of crops and other plants.